1. Field of the Invention
The present invention relates to an information input and output apparatus, more particularly, to an information input and output apparatus comprising an active matrix drive type display device and a light pen.
2. Description of the Related Art
As an information input and output apparatus, conventionally, there has been used an apparatus integrally comprising a separate CRT display device and a separate input apparatus for inputting information using a light pen. The apparatus of this type has a disadvantage that it is not suitable for portable use since the power consumption is relatively large and the size of the apparatus is relatively large.
In order to solve the above problems, there has been proposed in the Japanese patent laid open publication (JP-A) No. 61-6729 published on Jan. 13, 1986, an information input and output apparatus of a completely integral type comprising a combination of a liquid crystal display device and a light emitting type light pen. In the proposed information input and output apparatus, plural photoelectric transducers are arranged inside of an active matrix drive type liquid crystal display device so as to oppose respective pixel electrodes.
FIG. 1 shows a structure of a pixel of the conventional liquid crystal display device of this type.
Thin film transistors (referred to as TFTs hereinafter) for driving the pixel electrodes and photodiodes for detecting light projected from a light pen which are photoelectric transducers each having two terminals are formed on an electrically insulating substrate (not shown) in a matrix shape so as to oppose the pixel electrodes (not shown), respectively.
A TFT comprises a source line 63, a gate line 65, and a drain electrode 67, as shown in FIG. 1. In one pixel of the liquid crystal display device, in addition to these components, a photoconductive semiconductor film 62 for detecting light projected from the light pen is formed on the gate line 61 so as to correspond to each pixel electrode, and a data read line 69 is formed on the photoconductive semiconductor film 62 so as to solidly cross the gate line 61 in parallel to the source line 63.
FIG. 2 shows a conventional circuit for detecting a position of the light pen.
Referring to FIG. 2, after an image signal is inputted to a line memory 43 with a clock signal having a predetermined frequency and is stored therein, the image signal is read out from the line memory 43 and is outputted to each source line 63. Responsive to the clock signal, a scan pulse generator 81 generates a scan pulse and outputs it to the gate lines 61, which is connected to the data read line 69 through the photoconductive semiconductor film 62. The drain electrode of each TFT is connected to a capacitor Cs for storing a signal and each pixel electrode (not shown) which is connected to each opposing electrode (not shown) through the liquid crystal layer 110. Each data read line 69 is connected to one input terminal of each comparator 44, which compares the voltage of the data read line 69 with a reference voltage Vref so as to detect the position of the light pen when a pixel is indicated by the light pen so that a beam of light projected therefrom is incident onto one of photoconductive semiconductor films 62.
The information input and output apparatus of this type shown in FIGS. 1 and 2 has a relatively small power consumption, and can be miniaturized. However, it is necessary to form each data read line 69 in parallel to the source line 63, resulting in twice the number of the electrodes in the column direction of the matrix when each data read line 69 is not formed. In this case, the method of fabricating the liquid crystal display device of this type becomes extremely complicated, and the information input and output apparatus has such a disadvantage that it is extremely difficult to fabricate it. Further, since the structure of the apparatus of this type becomes complicated, there are such problems that an area of each opening on each pixel decreases and the contrast of the displayed image is lowered.